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      Regulatory T Cells in the Tumor Microenvironment and Cancer Progression: Role and Therapeutic Targeting

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          Abstract

          Recent years have seen significant efforts in understanding and modulating the immune response in cancer. In this context, immunosuppressive cells, including regulatory T cells (Tregs) and myeloid-derived suppressor cells (MDSCs), have come under intense investigation for their proposed roles in suppressing tumor-specific immune responses and establishing an immunosuppressive tumor microenvironment, thus enabling tumor immune evasion. Additionally, recent evidence indicates that Tregs comprise diverse and heterogeneous subsets; phenotypically and functionally distinct subsets of tumor-infiltrating Tregs could contribute differently to cancer prognosis and clinical outcomes. Understanding Treg biology in the setting of cancer, and specifically the tumor microenvironment, is important for designing effective cancer therapies. In this review, we critically examine the role of Tregs in the tumor microenvironment and in cancer progression focusing on human studies. We also discuss the impact of current therapeutic modalities on Treg biology and the therapeutic opportunities for targeting Tregs to enhance anti-tumor immune responses and clinical benefits.

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          CTLA-4 and PD-1 Pathways

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            Tumour hypoxia promotes tolerance and angiogenesis via CCL28 and T(reg) cells.

            Although immune mechanisms can suppress tumour growth, tumours establish potent, overlapping mechanisms that mediate immune evasion. Emerging evidence suggests a link between angiogenesis and the tolerance of tumours to immune mechanisms. Hypoxia, a condition that is known to drive angiogenesis in tumours, results in the release of damage-associated pattern molecules, which can trigger the rejection of tumours by the immune system. Thus, the counter-activation of tolerance mechanisms at the site of tumour hypoxia would be a crucial condition for maintaining the immunological escape of tumours. However, a direct link between tumour hypoxia and tolerance through the recruitment of regulatory cells has not been established. We proposed that tumour hypoxia induces the expression of chemotactic factors that promote tolerance. Here we show that tumour hypoxia promotes the recruitment of regulatory T (T(reg)) cells through induction of expression of the chemokine CC-chemokine ligand 28 (CCL28), which, in turn, promotes tumour tolerance and angiogenesis. Thus, peripheral immune tolerance and angiogenesis programs are closely connected and cooperate to sustain tumour growth. ©2011 Macmillan Publishers Limited. All rights reserved
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              Foxp3+ CD25+ CD4+ natural regulatory T cells in dominant self-tolerance and autoimmune disease.

              Naturally arising CD25+ CD4+ regulatory T (Treg) cells, most of which are produced by the normal thymus as a functionally mature T-cell subpopulation, play key roles in the maintenance of immunologic self-tolerance and negative control of a variety of physiological and pathological immune responses. Natural Tregs specifically express Foxp3, a transcription factor that plays a critical role in their development and function. Complete depletion of Foxp3-expressing natural Tregs, whether they are CD25+ or CD25-, activates even weak or rare self-reactive T-cell clones, inducing severe and widespread autoimmune/inflammatory diseases. Natural Tregs are highly dependent on exogenously provided interleukin (IL)-2 for their survival in the periphery. In addition to Foxp3 and IL-2/IL-2 receptor, deficiency or functional alteration of other molecules, expressed by T cells or non-T cells, may affect the development/function of Tregs or self-reactive T cells, or both, and consequently tip the peripheral balance between the two populations toward autoimmunity. Elucidation of the molecular and cellular basis of this Treg-mediated active maintenance of self-tolerance will facilitate both our understanding of the pathogenetic mechanism of autoimmune disease and the development of novel methods of autoimmune disease prevention and treatment via enhancing and re-establishing Treg-mediated dominant control over self-reactive T cells.
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                Author and article information

                Contributors
                Role: Academic Editor
                Journal
                Vaccines (Basel)
                Vaccines (Basel)
                vaccines
                Vaccines
                MDPI
                2076-393X
                06 August 2016
                September 2016
                : 4
                : 3
                Affiliations
                [1 ]Cancer Research UK Cambridge Institute, University of Cambridge, Cambridge CB2 0RE, UK; belal.chaudhary@ 123456cantab.net
                [2 ]Cancer Center, Qatar Biomedical Research Institute and College of Science and Engineering, Hamad Bin Khalifa University, Qatar Foundation, Doha 5825, Qatar
                [3 ]College of Medicine and Health Sciences, United Arab Emirates University, Al Ain 17666, UAE
                [4 ]Institute of Cancer Sciences, University of Manchester, Manchester M20 4BX, UK
                [5 ]Biomedical Research Centre, School of Environment and Life Sciences, University of Salford, Salford M5 4WT, UK
                Author notes
                Article
                vaccines-04-00028
                10.3390/vaccines4030028
                5041022
                27509527
                92ef36a9-d844-4cb8-bf2d-cc44a0aea79b
                © 2016 by the authors; licensee MDPI, Basel, Switzerland.

                This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license ( http://creativecommons.org/licenses/by/4.0/).

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